Yelp's universal stylesheets for Mallard and DocBook Formula was removed
Last version was
yelp-xsl requires the following formulae to be installed:
xz 5.2.3 General-purpose data compression with high compression ratio
itstool 2.0.4_1 Make XML documents translatable through PO files
intltool 0.51.0 String tool
gettext GNU internationalization (i18n) and localization (l10n) library
libxslt 1.1.32 C XSLT library for GNOME
libxml2 2.9.7 GNOME XML library

Recent formula history

A. Jesse Jiryu Davis yelp-tools: convert yelp-xsl to a resource.
Dominyk Tiller yelp-xsl 3.20.1
Tom Schoonjans yelp-xsl 3.18.1
Tom Schoonjans yelp-xsl 3.18.0
Mike McQuaid yelp-xsl 3.16.1 (new formula)

Formula code at GitHub